This invention relates generally to signal generators, and more particularly to those in the microwave frequency range that are rapidly switchable between specific frequencies for, as an example, simulating a field including numerous distinct frequency radar signals.
A phase locked loop is a commonly used element for a variable frequency signal generator. Frequency output can be changed among discreet frequencies within the output frequency range. The frequency range of a phase locked loop, however, is limited by the range of frequencies which its components can handle. If a signal generator with a wider frequency range is desired, a plurality of phase locked loops may be utilized, each handling a distinct portion of the expanded frequency range, but this is complicated and expensive. Also, phase locked loops of the type presently utilized are slow in changing frequency.
Another technique for a broad frequency band signal generator is to utilize a separate fixed frequency oscillator for each of the discrete frequencies that are desired to be generated within the band. This, of course, involves a great deal of circuit elements and thus is complicated and expensive. For example, if a microwave signal generator has a range of 0.5 GHz to 18.0 GHz by frequency steps of 10 MHz, a total of 130 oscillators at distinct frequencies would be required. Existing techniques suggest that fewer oscillators could be used by selectively combining the oscillators to provide the desired discrete frequency steps but this is also complex and cumbersome if a wide frequency spectrum is desired.
One application of such a signal generator is in the simulation of a radar field that contains a number of different radar signals. Such signal generators are used for training, as an example, of military or other personnel that have a job of detecting and analyzing radar signals. Rather than having separate signal generators for each simulated radar signal, it involves considerably less hardware to utilize one broad band microwave signal source that is rapidly switched between frequencies. After one microwave signal pulse at one particular frequency is emitted then there is a time delay before a second pulse at that frequency needs to be emitted in simulating the radar signal. In this interim, the same oscillator can be used to generate one or more additional microwave pulses at other distinct frequencies to simulate additional radar signals in the field. To do this, however, requires extremely fast switching between frequencies.
Therefore, it is a principal object of the present invention to provide a wide band microwave frequency generator that may be rapidly switched between specific frequencies within its frequency band, is stable in frequency output over time and yet is relatively simple so that the cost and amount and size of hardware can be minimized.